Major human airway diseases are characterized by excessive airway mucus and infection so that new hypotheses for airway mucus formation and its relationship to infection are required to understand and treat this type of disease. This projet concentrates on epithelial cell production of IL-33 as a critical regulator of excess mucus production in airway disease. The focus derives from findings in both a mouse model of excess mucus production due to viral infection and in translational studies of humans with excess mucus production due to COPD. In the mouse model, parainfluenza virus (PIV) infection leads to production of IL-33 and in turn innate immune cell production of IL-13 and consequent overproduction of airway mucus, and this process is enhanced by tobacco smoking. IL-33 production is traceable to a subpopulation of epithelial cells that may be linked to cell renewal, repair, and remodeling. In humans with COPD, IL-33 production is also increased in concert with up-regulation of IL-13 and airway mucus production. In this case, increased IL-33 production is traceable to a subpopulation of basal progenitor cells that maintain an endogenous capacity for increased pluripotency and ATP-dependent release of IL-33 even ex vivo. We therefore propose that a sustainable (progenitor) epithelial cell population (particularly basal-lie cells in humans) may be activated by epithelial danger signals (particularly ATP) to release IL-33 and thereby lead to excess airway mucus production. The progenitor nature of this IL-33-expressing ATP-responsive cell population could explain an acquired susceptibility to excess mucus production. The findings may therefore provide a new paradigm to explain the role of tobacco smoking and viral infection in the excess mucus production of chronic airway disease. Our preliminary studies lead to the following specific aims: 1. In mouse models: Define the functional cell sources and targets of IL-33 that underlie excess airway mucus production in a postviral mouse model with or without tobacco smoking in vivo and establish the existence of an IL-33-producing/releasing epithelial progenitor cell population using this model and the corresponding mouse cells studied in vitro. 2. In humans: Establish the existence of an IL-33-expressing/releasing progenitor cell population linked to excess mucus production in patients with COPD at baseline and during virus-induced exacerbation in vivo (using clinical samples) and in vitro (using epithelial cells isolated from these samples). These studies will test our proposal for IL-33 expression and release from a specific epithelial progenitor population that exhibits increased capacities for self-renewal, IL-33 release, and mucous cell differentiation, and thereby contributes to a vicious cycle wherein smoking and infection lead to chronic excess mucus production.